The field of art to which this invention pertains is catalysts for the selective hydrogenation of polyunsaturated hydrocarbons.
Catalysts made with palladium and a promoter on a metal oxide support have been used for the selective hydrogenation of butadiene, pentadiene and cyclopentadiene as described, for example, in West German Patent Applications, DE-A-3,207,029, DE-A-3,207,030 and DE-A-3,143,647, and in European Patent Application EP-A 11,906. In these cases, the hydrogenation reactions were carried out predominantly with catalysts made from palladium supported on Al.sub.2 O.sub.3 or SiO.sub.2 in particular. The conduct of the reaction is dealt with in detail in a paper by H. Laufer in "Erdol and Kohle" ("Petroleum and Coal"), volume 36, 249 (1983). A paper by M. L. Derrieu in "Catalytic Hydrogenation", Elsevier, Amsterdam, 1986, page 613, also describes established commercial processes and the catalyst used therein. The selectivity of these known catalysts is, however, still unsatisfactory.
A marked improvement of the selectivity in the case of diene or acetylene hydrogenation (increased olefin yield) was achieved by a partial poisoning (promotion of Pd catalysts with Zn, Cd, Sn, Pb and Hg; see G. C. Bond, "Catalysis by Metals", Academic Press, London, 1962, pages 99 and 297). The use of CaCO.sub.3 as support and promotion of the palladium with Zn also led to an increase in selectivity in the case of the cyclopentadiene hydrogenation (U.S. Pat. Nos. 3,947,510 and 3,857,894). The use of other supports, such as MgO or BaSO.sub.4, is likewise known in the case of Pd catalysts (R. J. Peterson, "Hydrogenation Catalysts", Noyes Data Corp., New York, 1977, page 183).
The selective hydrogenation of dienes containing more than five carbon atoms, for example from C.sub.6 to C.sub.10 dienes, in particular with isolated double bonds, however, becomes particularly difficult. During hydrogenation, they usually behave as monoolefins, i.e. they are immediately perhydrogenated to form the corresponding alkanes. If it is desired to achieve a selective hydrogenation which stops at the monoolefin stage, the reaction should be preceded, according to the opinion of the art, by a double bond isomerization to form a conjugated diene. Said isomerization is affected, as a rule, by the surface properties of the support material, for instance its acidity.
For hydrogenating longer-chain dienes, the literature frequently describes nickel catalysts which are partially poisoned with sulfur to increase the selectivity. The hydrogenation, described in U.S. Pat. No. 3, 472,763, of a pyrolysis gasoline containing a proportion of diolefins of 6.8% by volume may serve as an example. Disadvantages of said catalyst are high hydrogenation pressure and low velocities, for example about 40 bar and 2 liters of hydrocarbon (liquid) per litre of catalyst and hour.
More suitable are the catalysts for the selective hydrogenation of polyunsaturated organic compounds known from West German Patent Application DE-A-3,320,388. Said catalysts are characterized by
(A) a metal component composed of one or more elements of group VIII of the periodic system, in particular palladium,
(B) a support material based on
(b.sub.1) one or more n-type semiconducting oxides of one or more elements from subgroups IVb (in particular TiO.sub.2), Vb and VIb of the Periodic Table or of thorium or cerium, or based on PA0 (b.sub.2) one or more n-type semiconducting mixed oxides of the formula Me.sub.2 Me.sub.1 (O).sub.x, wherein Me.sub.1 denotes an element from the group defined in (b.sub.1), Me.sub.2 denotes an alkaline-earth metal or an element, different from Me.sub.1, from the group defined in (b.sub.1) and x is the number of oxygen atoms required to saturate Me.sub.1 and Me.sub.2 so as to reach the range of an n-type semiconducting state;
the hydrogen chemisorption power, expressed as the atomic ratio between chemisorbed hydrogen atoms and metal atoms of the metal component (A) (H/Me.sub.A) at the surface of the metal particles, being at least 0.6:1.
The exemplary embodiments of DE-A-3,320,388 describe the hydrogenation of C.sub.6 and C.sub.10 dienes in a 1:10 mixture with the corresponding monoolefins.
As the molecular weight of the dienes increases and the concentration in the hydrocarbon fractions decreases, the selective hydrogenation is, however, made markedly more difficult. Such hydrocarbons originate, for example, from thermal or catalytic cracking processes and catalytic dehydrogenations. During the catalytic dehydrogenation of hydrocarbons containing more than five up to about thirty carbon atoms, for instance from C.sub.10 to C.sub.15 paraffins, in order to produce higher mono-olefins, a product mixture is produced for example, composed of non-reacted paraffins with about 10% mono-olefins and about 1% diolefins. The proportion of diolefins has to be reduced in view of the undesired side reactions during the further processing. A selective hydrogenation of the diolefins with an increase in monoolefins imposes particularly high requirements on the catalyst in such product flows.
Apart from high selectivity, the interposing of a hydrogenation stage is only economically acceptable if the hydrogenation catalyst makes possible short residence times, a lower hydrogenation pressure and the use of moderate temperatures.
The invention is consequently based on the object of improving the catalysts described in DE-A-3,320,388 in relation to their range of application (low hydrogenation pressures and temperatures) and also in relation to their selectivity.